1). Zero no Tsukaima (The Familiar of Zero) - the combination of magic, history and modern technology did it for me. Also got me into writing isekai. It's also an annual tradition for me to rewatch the four seasons every Christmas Day.
2). Ookami to Shinkuryo (Spice and Wolf) - the show taught me about business and blending real life mechanics with fantasy elements especially the "boring" stock market.
3). ToraDora - I always recommend this to my students, mainly for its message.
4). Log Horizon - I like this show more than its contemporary Sword Art Online, mainly because Shiro made more sense to me than Kirito.
5). Shield Bro - I like his progression, though what kind of turned me off is the MC's edginess. Still in my Top 5 though I read the web novel instead of actually watching the anime.
Honorable Mentions
Mushoku Tensei - my primary turn off is the MC, though he did a character development to the end (I also read the web novel instead of the anime). MT also made me realize that the concept of monogamy lies only in the culture, not the 'absolute' morality of the society.
Haruhi Suzumiya - yeah, I was influenced by its unorthodoxy and whimsicality, which I still apply in my life today.
Kaze no Stigma - strong start, but author died, so...
Hundred Stories - I'm a sucker for Japanese horror blended with creepy animation, so this one's good stuff for me.